If, like myself, one reason you enjoy kdramas over western is the more innocent relationship portrayal, then yeah this won't be for you. It's a fairly good plot made better by the somewhat unique dual portrayal, but this is far more sexual than anything I have ever watched in western film (granted I try to stay away from that so)... Definitely 18+.
It felt really weird after watching Kim Tae Ri in Mr. Sunshine and then seeing this a few months later; actually, it was almost a little disturbing. I think it could have been one of the better Korean movies I've seen if it did not have the sexual content, though they probably couldn't have kept the plot in that case. I understand the differences in Korean movie vs TV but I have grown used to other very good Korean Movies that were very mature in gore and barely touched on anything sensual; I much prefer that. Otherwise, I'll stick the 15+ dramas.

Story (6.5):
The story is problematic to me mostly because it all revolves around sensuality and it cast a tone to the movie I couldn't truly enjoy. If that is more your thing, then it does have a fairly intricate and exciting plot.
That said, I believe most of my enjoyment in the story came from the way it was portrayed, as I mentioned earlier. I don't want to give any spoilers, but I'll just say that there are 3 parts to the movie and they don't reveal everything that happens in part 1.
Without this method of storytelling I think the plot could have been just a jumble, so it's a good thing it was done this way; props to the screenwriter, who coincidentally also worked on the Korean adaptation of Mother which is one of my all time favorite dramas.
We all also know the director but I'm not the greatest fan of his works though Oldboy is irrefutably a well done movie. If I could enjoy the theme, then I believe the story portion would get a higher rating, but I can't handle it. Sue me, to each their own.

Acting/Cast (10.0):
Of course, this is a phenomenal cast. I don't think I can say much beyond it was very well acted. Kim Tae Ri is one of my favorite actresses after seeing Mr. Sunshine, and to be honest, after seeing the story prior to watching, the only reason I still went forward with my viewing was because of her. Everyone else in the cast was very well done, especially the Count and the Uncle. I've not seen much of Kim Min Hee, but nothing to complain about from there; obviously a very seasoned actress. I think one of the most impressive parts is that everyone seems so fluent in Korean and Japanese!

Music (5.0):
Often I'll give the music portion a high rating if I felt it wasn't unique or especially good but fit with the drama and didn't detract at all. I have to say this did a fairly good job, but there were 1 or 2 instances where I frowned at what I was hearing. It was still pretty tasteful and there was a fair amount without music, but I heard nothing especially interesting.

Rewatch value (1.0):
So yeah, I'm never going to watch this again.

Overall (5.5):
Is anyone still with me? At the end of the movie, I was fairly pleased with the progression and then woah here we go again... you'll know what I mean. Like seriously did you have to do that... I'm scarred for life.
If you think I'm harsh because blah blah I can't handle it, okay fine, I was home-schooled and grew up on Winnie the Pooh and Mr. Rogers so sue me. I remember the first time I realized Korean entertainment wasn't all prudish and fade to black was when I watched Little Girl K and there was suddenly a bared breast. It literally shocked me; the reason I'd grown so attached at that point to Kdramas was because, after just moving out of my parent's house, I had marathoned and quickly grown disillusioned with a ton of western TV shows and movies with only a handful of ones I actually enjoyed. Compare to what I found when stumbling upon Kdramas; it was like a whole new pasture of green grass without a thistle in sight. There were plenty of Kdramas I didn't find very interesting, but I didn't drop them because the content was literally abhorrent and stupid (except Boys over Flowers).
In the end I actually did enjoy Little Girl K because the nudity was far from the main content and wasn't really dwelt on either. Some of my favorite western movies are ones rated R with no sexual content in them and I found that almost everything I came upon in the Korean world had enough maturity and gore to not be a stupid teen show (gag at almost every single western teen show, not just the bad acting...) but was almost without exception less sexual than any of the aforementioned western teen shows.
I can still remember patiently watching 21 episodes of Shining Inheritance to be rewarded with a kiss! Now that's what I'm talking about!